US3054259A

US3054259A - Combustion apparatus

Info

Publication number: US3054259A
Authority: US
Publication date: 1962-09-18
Anticipated expiration: 1979-09-18

Description

p 1962 J. R. ARTHUR ETAL 3,054,259

COMBUSTION APPARATUS Filed Feb. 14

Inventors- James R Arthur, Robert E owers,

by 11 Their Attorney United States Patent 3,054,259 COMBUSTION APPARATUS James R. Arthur and Robert F. Powers, Scotia, N.Y., as-

signors, by mcsne assignments, to the United States of America as represented by the Secretary of the Army Filed Feb. 14, 1951, Ser. No. 210,939 1 Claim. (Cl. 60-39.72)

This invention relates to combustion apparatus. More particularly, it relates to combustion apparatus embodying a new and useful flameholder, said apparatus being characterized by easy ignition, stability of combustion over a wide range of fuel-oxidizer velocities as well as enhanced combustion of the fuel.

One of the primary problems in combustion apparatus having a relatively high reactant velocity is that of maintaining the flame or preventing it from being extinguished. The problem is particularly acute in the case of gas turbines and jet propulsion devices such as ram jets or athodyds, among others, wherein fuel-oxidizer or fuel-air flows up to and in excess of 500 feet per second are encountered. The difiiculty in maintaining a flame in high velocity streams stems from the fact that the speed at which the flame propagates in the usual fuel-oxidizer mixtures is comparatively very low. For example, at ordinary ambient temperatures the flame from a stoichiometric hydrocarboneir mixture has a velocity of about one foot per second.

An additional problem arises from the fact that under such high velocity gas stream conditions, the flame tends to be carried out of the reaction chamber before combustion has had an opportunity to spread throughout the chamber and completely burn the fuel-oxidizer mixture.

The above-mentioned problems, that of a tendency toward flame extinguishment and of incomplete combustion experienced in high velocity fuel-oxidizer streams, have seriously detracted from the efflciency and utility of combustors which operate under conditions of high reactant velocity.

It is an object of the present invention to provide an improved flameholder for combustion apparatus.

Another object of the present invention is to provide combustion apparatus which is characterized by flame stability and completeness of combustion.

A further object of this invention is to provide combustion apparatus which is characterized by flame stability and complete combustion over a wide range of fueloxidizer velocities and mixtures.

Other objects will become apparent and the invention better understood from a consideration of the following description and the drawing in which FIG. 1 shows a configuration of the new flameholder installed in a combustor duct, FIGS. 2 and 3 are typical cross-sectional views of portions of the flameholder embodiments and FIG. 4 is an elevational view from the upstream direction of another configuration of the flameholder.

It has been found that the flame in a combustor may be maintained with high velocity reactant flow by providing in the path of the fuel-oxidizer or fuel-air mixture a flameholder of unique design.

More particularly, it has been found that the flame may be maintained under such conditions and more complete combustion attained by providing in the combustor a flameholder having a generally U-shaped upstream-downstream cross-section with the open end disposed downstream.

Specifically, it has been found that the flameholder shoulder have a generally U-shaped cross-section with the sides thereof parallel or slightly spread, the upstream closed end of the flameholder having holes therein for the ingress of the fuel-air mixture and a perforated bafile disposed within said flameholder perpendicular to the downstream direction. Igniting mean for the fuel-air mixture are provided in the flameholder.

The principle upon which the new flameholder is based is as follows. The fuel-air mixture progressing downstream impinges upon the closed upstream face of the flameholder. Since the sides of the flameholder are either parallel or slightly spread apart in the downstream direction, the static pressure at the trailing edges is less than that immediately upstream of the holder. This pressure ditlerential facilitates the entrance of the fuel-air mixture into the gutter-shaped flameholder through the holes in the upstream closed edge over and above that which wouhl enter by reason of impingement alone. Were the sides of the holder drawn together rather than being parallel or slightly spread in the downstream direction, this pressure differential would not exist and the combustion mixture would enter the holder only by reason of impingernent.

The perforated battle within the flameholder serves to diffuse the fuel-air mixture. This baflie may take various forms. While it is preferably a perforated metal sheet, wire mesh or other structures may be employed. In general, the open space to give proper diffusion ranges from fifteen percent to fifty percent of the total baflle area. The total frontal or chamber blocking area of the flameholder is from about twenty-five percent to about forty percent of the total cross-sectional area of the combustion space. Using structures having the above proportions, positive ignition and maintenance of flames are obtained at fuel-air ratios varying from 0.01 to 0.07 at velocities up to over five hundred feet per second.

Preferably the frontal area of the flameholder is about forty percent relative to the combustion cross-section and the open space in the internal baflle about thirty percent of the measured baffle area. The specific preferred fuelair ratio is 0.04.

Ignition of the fuel-air mixture is obtained by the use of a sparking device such as a spark plug downstream of the flameholder bathe, only one such sparking unit being essential for any flameholder configuration.

For a better understanding of the new flameholder and its application, attention is invited to the drawing in which FIG. 1 illustrates a typical flameholder configuration. Mounted centrally within combustor or combustion duct 1 is the new flameholder 2. The particular configuration is in the shape of a cross which for smaller ducts insures complete ignition of the combustible mixture by means of sparking device 3, as well as steady maintenance of the flame front over a wide range of fuel-air ratios and velocities.

The cross-section of the flameholder 2 is shown in more detail in FIG. 2. The closed end or leading edge of the holder is rounded to provide a minimum of resistance to the fluid flow and the sides are parallel one to the other. In order to provide for the entry of the fuel-air mixture into the holder a plurality of holes 4 and 5 are provided in the leading edge. Located downstream of the leading edge and within the holder is bafile 6 which is disposed parallel to the leading edge and, as pointed out above. provided with a plurality of holes which serve to diffuse the flow and further reduce the velocity of the mixture. Generally the open space represented by the holes in the baffle ranges from fifteen percent to fifty percent of the total frontal area of the baflle. While there is no general limitation as to the structural proportions of the flameholder itself, good results have been obtained with the proportions shown in which the baflle height is roughly one-third of the total upstream-downstream length. The bafiie may be placed at any point in the holder, but is preferably placed one-quarter to one-third of the total length downstream from the leading edge. Such location appears to allow suflicient diffusion and furnishes a quiescent zone in which the flame remains stable. The cornbustible fluid is ignited downstream of the bathe 7 by a spark from sparking device 3, the burning within the holder being relatively quiet with a steady non-extinguishing flame extending downstream of the holder to maintain constant ignition of the combustible mixture throughout the cross-section of the combustion chamber.

An alternative cross-sectional configuration of the flameholder is shown in FIG. 3. Here the sides of the holder are spread outwardly downstream providing a relatively lower downstream static pressure than the type of FIG. 2 and hence a greater flow of combustible mixture through the holder. However, here again the blockage of the combustion duct or chamber should not exceed about forty percent. Air is introduced into the duct by any well known means while any usual type of nozzle 8 may be used to feed the fuel. Furthermore, the distance of the holder from the air and fuel entry points is not critical.

A simple cross-shaped flameholder such as shown in FIG. 1 is adequate to insure complete and steady combustion of fuel-air mixtures in ducts or chambers ranging up to about six inches in diameter at fuel-air ratios of 0.01 to 0.07 and velocities up to over 500 feet per second. For larger diameter chambers other configurations may be readily devised by arranging the holder in circular form and with or without spokes or spoke extensions as the particular design dictates. For example, in FIG. 4 is shown a configuration for a chamber about eighteen inches in overall diameter. Here the flameholder is mounted within chamber 10 and assumes a circular shape with extended spokes which assures complete and steady combustion of the combustible fuel throughout the chamber cross-section once sparking device 11 has initiated combustion within the holder.

The new flameholder is not only for use in combustion chambers burning fuel with air having a normal oxygen content, but in air having an oxygen content down to about fifteen percent. Thus, the flameholder is useful in the afterburners of jet propulsion engines.

Any fuel ranging from Bunker C and such heavy oils to the more highly refined oils and fuels, including the gasolines, may be used with the new flameholder.

The usual materials of construction may be utilized in making the new flameholder. These include steel, stainless steel, heat resistant chromium-nickel-iron alloys and many others which are known to those versed in the art.

There is provided by the present invention a flameholder which is useful in initiating and maintaining combustion of fuel-air mixtures in combustion chambers over a wide range of mixture velocities and fuel concentrations. Positive ignition is obtained without the use of pilot burners. The combustion process is further characterized by quietness as well as stability and completeness of reaction.

While only certain embodiments of the invention have been described herein, it will be understood that it is intended to include all those modifications which do not depart from the spirit and scope of the invention.

What We claim as new and desire to secure by Letters Patent of the United States is:

Combustion apparatus comprising a duct, means to supply air and fuel to said duct, and a flameholder downstream of said supply means, said flameholder comprising a generally U-shaped structure with a rounded generally closed leading edge and with its open end downstream, the upstream-downstream sides of said structure being so disposed that under flow conditions the static pressure immediately downstream of said structure is lower than that immediately upstream of said structure, said flameholder having a total frontal area ranging from about twenty five to forty percent of the crosssectional area of said duct, said leading edge having holes therein for the ingress of combustion reactants, a perforated baffie within said structure to diffuse the reactant flow, the open area of said bafile comprising about fifteen to fifty percent of the total area of said bafiie, the baffle height being approximately one-third of the length of said upstreamdownstream sides and being located about one-quarter to one-third of said length from said leading edge, and reactant igniting means within said flameholder downstream of said bafiie.

References Cited in the file of this patent UNITED STATES PATENTS 2,556,161 Bailey et a] June 12, 1951 2,679,137 Probert May 25, 1954 2,701,444 Day Feb. 8, 1955 OTHER REFERENCES Vincent: Theory and Design of Gas Turbines and Jet Engines, McGraw-Hill Book Co., 1950, pages 507-515.